SLG4: initial work FileSaver RenderEngine

--HG--
branch : slg3.5_startup
rename : samples/smallluxgpu4/pathcpu/pathcpu.cpp => samples/smallluxgpu4/filesaver/filesaver.cpp
rename : samples/smallluxgpu4/pathcpu/pathcpu.h => samples/smallluxgpu4/filesaver/filesaver.h
SLG4: fixed a recently introduced bug in Null material

--HG--
branch : slg3.5_startup
SLG4/MacFix: fixed a bug in Null material, Test #2

--HG--
branch : test_mac_fix_for_slg4